The invention relates to a process for upgrading Fischer-Tropsch products by increasing the yield of diesel and lubricating base oil produced from a Fischer-Tropsch plant.
The market for lubricating base oils of high paraffinicity is continuing to grow due to the high viscosity index, oxidation stability, and low volatility relative to viscosity of these molecules. The products produced from the Fische-Tropsch process contain a high proportion of wax which make them ideal candidates for processing into lubricating base oil stocks. Accordingly, the hydrocarbon products recovered from the Fischer-Tropsch process have been proposed as feedstocks for preparing high quality lubricating base oils. See, for example, U.S. Pat. No. 6,080,301 which describes a premium lubricating base oil having a high non-cyclic isoparaffin content prepared from Fischer-Tropsch waxes by hydroisomerization dewaxing and solvent dewaxing. Lubricating base oils typically will have an initial boiling point above about 315 degrees C. (600 degrees F.). High quality diesel products also may be prepared from the syncrude recovered from the Fischer-Tropsch process. Fischer-Tropsch derived diesel typically has a very low sulfur and aromatics content and an excellent cetane number. These qualities make Fischer-Tropsch derived diesel an excellent blending stock for upgrading lower quality petroleum-derived diesel. Accordingly, it is desirable to be able to maximize the yields of such higher value hydrocarbon products which boil within the range of lubricating base oils and diesel. At the same time, it is desirable to minimize the yields of lower value products such as naphtha and C4 minus products.
All syncrude Fischer-Tropsch products as they are initially recovered from the Fischer-Tropsch reactor contain varying amounts of olefins depending upon the type of Fischer-Tropsch operation employed. In addition, the crude Fischer-Tropsch product also contains a certain amount of oxygenated hydrocarbons, especially alcohols, which may be readily converted to olefins by a dehydration step. These olefins may be oligomerized to yield hydrocarbons having a higher molecular weight than the original feed. Oligomerization also introduces desirable branching into the hydrocarbon molecule which lowers the pour point of the diesel and lubricating base oil products, thereby improving the cold flow properties of the product. See for example U.S. Pat. No. 4,417,088.
Fischer-Tropsch wax refers to a high boiling fraction from the Fischer-Tropsch derived syncrude and is most often a solid at room temperature. For the purpose of this disclosure xe2x80x9cFischer-Tropsch waxxe2x80x9d will be contained in the higher boiling portion of the Fischer-Tropsch syncrude. Fischer-Tropsch wax contains at least 10% by weight of C20 and higher hydrocarbonaceous compounds, preferably at least 40% by weight of C20 and higher hydrocarbonaceous compounds, and most preferably at least 70% by weight of C20 and higher hydrocarbonaceous compounds. Fischer-Tropsch wax is important for the present invention because this fraction will contain the heavier hydrocarbons which will be sent to the catalytic dewaxing operation. Depending on how the operation is run, the majority of the Fischer-Tropsch wax may be converted to high quality lubricating base oil and diesel.
As used in this disclosure, the term xe2x80x9cC19 minus Fischer-Tropsch productxe2x80x9d refers to a product recovered from a Fischer-Tropsch reaction zone which is predominantly comprised of hydrocarbons having 19 carbon atoms or less in the molecular backbone. One skilled in the art will recognize that such products may actually contain a significant amount of hydrocarbons containing greater than 19 carbon atoms. In general, what is referred to are those hydrocarbons having a boiling range of diesel and below. In general, for the purposes of this disclosure, diesel is considered as having a upper boiling point of about 700 degrees F. (370 degrees C.) and an initial boiling point of about 300 degrees F. (about 150 degrees C.). Diesel may also be referred to as C10 to C19 hydrocarbons. Likewise, Fischer-Tropsch wax preferably is comprised predominantly of xe2x80x9cC20 plus productxe2x80x9d which refers to a product comprising primarily hydrocarbons having 20 or more carbon atoms in the backbone of the molecule and having an initial boiling point at the upper end of the boiling range for diesel, i.e., above about 600 degrees F. (315 degrees C.). It should be noted that the upper end of the boiling range for diesel and the lower end of the boiling range for Fischer-Tropsch wax have considerable overlap. The term xe2x80x9cnaphthaxe2x80x9d when used in this disclosure refers to a liquid product having between about C5 to about C9 carbon atoms in the backbone and will have a boiling range generally below that of diesel but wherein the upper end of the boiling range will overlap that of the initial boiling point of diesel. The term C10 plus hydrocarbons refers to those hydrocarbons generally boiling above the range of naphtha, i.e., the fractions boiling within the range of diesel and lubricating base oils or above about 150 degrees C. Products recovered from the Fischer-Tropsch synthesis which are normally in the gaseous phase at ambient temperature are referred to as C4 minus product in this disclosure. LPG which is primarily a mixture of propane and butane is an example of a C4 minus product. The precise cut-point selected for each of the products in carrying out the distillation operation will be determined by the product specifications and yields desired.
As used in this disclosure the words xe2x80x9ccomprisesxe2x80x9d or xe2x80x9ccomprisingxe2x80x9d is intended as an open-ended transition meaning the inclusion of the named elements, but not necessarily excluding other unnamed elements. The phrase xe2x80x9cconsists essentially ofxe2x80x9d or xe2x80x9cconsisting essentially ofxe2x80x9d is intended to mean the exclusion of other elements of any essential significance to the composition. The phrases xe2x80x9cconsisting ofxe2x80x9d or xe2x80x9cconsists ofxe2x80x9d are intended as a transition meaning the exclusion of all but the recited elements with the exception of only minor traces of impurities.
The present invention is directed to a process for increasing the yield of C10 plus hydrocarbon products from a Fischer-Tropsch plant which comprises (a) recovering separately from a Fischer-Tropsch reactor a Fischer-Tropsch wax fraction and a Fischer-Tropsch condensate fraction, wherein the Fischer-Tropsch condensate fraction contains alcohols boiling below about 370 degrees C.; (b) dewaxing the Fischer-Tropsch wax fraction in a catalytic dewaxing zone to produce a high boiling intermediate having a lower pour point as compared to the Fischer-Tropsch wax fraction; (c) hydrofinishing the high boiling intermediate in a hydrofinishing zone; (d) contacting the Fischer-Tropsch condensate fraction separated in step (a) with a dehydration catalyst in a dehydration zone, whereby at least some of the alcohols present in the fraction are converted to olefins; (e) oligomerizing the olefins in the Fischer-Tropsch condensate fraction to form a intermediate oligomerization mixture having a higher average molecular weight than the Fischer-Tropsch condensate fraction; (f) hydrofinishing the intermediate oligomerization mixture in the hydrofinishing zone; and (g) and recovering from the hydrofinishing zone a C10 plus hydrocarbon product. The term Fischer-Tropsch condensate fraction refers generally to that C5 plus fraction which has a lower boiling range than the Fischer-Tropsch wax fraction. That is to say, that fraction which is normally liquid at ambient temperature.
In the prior embodiment the alcohols in the Fischer-Tropsch condensate undergo a dehydration step to convert them into olefins. In an alternative embodiment, the entire Fischer-Tropsch syncrude is sent to a pre-treatment operation which includes the dehydration step prior to separation of the syncrude into the Fischer-Tropsch wax fraction and the Fischer-Tropsch condensate fraction. Accordingly, the process may also be described as a process for increasing the yield of C10 plus hydrocarbon products from a Fischer-Tropsch plant which comprises: (a) contacting a feedstock comprising C5 plus products and alcohols recovered from the Fischer-Tropsch plant with a dehydration catalyst under dehydration conditions to convert at least some of the alcohols present into olefins; (b) separately recovering from the dehydrated feedstock a Fischer-Tropsch wax fraction and a Fischer-Tropsch condensate fraction comprising both saturated hydrocarbons and olefins having an upper boiling point below about 370 degrees C.; (c) dewaxing the Fischer-Tropsch wax fraction in a catalytic dewaxing zone to produce a high boiling intermediate having a lower pour point as compared to the Fischer-Tropsch wax fraction; (d) hydrofinishing the high boiling intermediate in a hydrofinishing zone; (e) oligomerizing the olefins in the Fischer-Tropsch condensate fraction recovered in step (b) to form a intermediate oligomerization mixture having a higher average molecular weight than the Fischer-Tropsch condensate fraction; (f) hydrofinishing the intermediate oligomerization mixture in the hydrofinishing zone; and (g) and recovering from the hydrofinishing zone a C10 plus hydrocarbon product. In this embodiment, it may be desirable to also include the removal of various catalyst poisons during the pretreatment of the feedstock. In this embodiment, some normally gaseous olefins, such as ethylene, propene, and butene, also may be present and will undergo oligomerization to yield higher molecular weight products.
The present invention is particularly advantageous for producing high quality lubricating base oils. A special benefit of the presentprocess is that it may be used to produce bright stock which is a high viscosity, highly refined, and dewaxed premium product. In conventional petroleum operations bright stock is produced from residual stocks or bottoms and has a high commercial value. Bright stock is named for the SUS viscosity at 210 degrees F., having viscosities above 180 cSt at 40 degrees C., more preferably above 250 cSt at 40 degrees C., and still more preferably ranging from about 500 to 1100 cSt at 40 degrees C.
The dewaxing of the Fischer-Tropsch wax is preferably carried out as a catalytic hydroisomerization step in order to minimize the wax cracking which reduces the yield of the higher molecular weight products. Accordingly, the use of hydroisomerization catalysts, such as SAPO-11, in combination with a noble metal, such as platinum or palladium, are particularly preferred for use in the dewaxing operation.